1. Field of the Invention
The present invention relates generally to an apparatus for cooling a heated strip of material such as a metal. The invention may be advantageously employed in such operations as, for example, a hot dip galvanizing line. The heated material is cooled by an apparatus which provides a uniform discharge of low-pressure, high velocity air on the material as the material travels adjacent the apparatus.
2. Description of the Prior Art
The coating of traveling sheet stock is old in the art. For example, galvanizing operations of steel sheet stock has been developed to prevent oxidation of the sheet metal later used to manufacture finished consumer products, such as garbage cans, air ductwork, storage tanks, and the like.
Generally, the galvanizing operations consist of coating the strip by dipping it in a molten bath of zinc, an alloy of zinc and one or more other metals, and optionally, other minor additives. The zinc coating operations have evolved from a single sheet (i.e., batch) process to the present-day continuous coating lines, commonly known as "CC lines". Generally, the CC lines in use today include five process steps. The first step involves a cleaning process which prepares the steel strip (for example, by degreasing) for coating. The second step is an annealing process, which endows the steel strip with good formability. The third step is a dipping process, in which the steel strip is dipped into a zinc bath to provide the protective coating. The fourth step is a chemical treating process, which protects the zinc coating from storage stains. The fifth step is the working and leveling process, which ensures uniform forming.
When the steel strip is zinc-coated in the hot dip step, the preheated strip generally travels downwardly from an annealing furnace into a bath of molten zinc where the surfaces of the strip are coated. The steel strip is thereafter turned upwardly by a sink, or guide, roller and leaves the molten zinc bath vertically. On leaving the bath, the strip is passed between opposed surfaces of air knives. The air knives provide a continuous blast of highpressure air to remove any excess molten zinc from the strip. At the time the coated strip exits the air knives, the zinc coating on the strip is still in a substantially liquid (i.e., molten) state.
As the strip continues traveling upwardly toward a deflector roller, often referred to as a tower roller, the temperature of the ambient air cools the strip and the zinc coating solidifies. During this solidification, the characteristic surface crystal design pattern, called "spangle", routinely seen on galvanized articles is formed. The rate of cooling determines the spangle pattern; if cooling is strictly by ambient air, large spangle patterns are formed. Galvanized products having large spangle patterns have a limited market for products in those areas where surface texture is unimportant. However, for consumer products, such as home appliances, office furniture, auto bodies, steel siding for homes and manufacturing facilities and the like, surface texture is an important feature. Many of these consumer products require a paint or other type of finish in addition to the galvanizing process. The large spangle pattern often shows through such finish as surface imperfections and is, thus, not desirable for many consumer product uses.
To prevent large spangle patterns from forming, attempts have been made by galvanizers to cool quickly the zinc and solidify it in a sudden manner by spraying air-atomized water on the strip exiting the air knives. This approach creates smaller spangle patterns; these smaller patterns are commonly referred to as "mini-spangles". However, even the "mini-spangles" are unacceptable in some consumer products due to the appearance of surface imperfections on the finished surfaces.
In related applications, the hot dip galvanizing industry introduced the use of an annealing furnace positioned immediately above the air knives. The purpose of this annealing furnace is to alloy the zinc and its additives to the material of the strip while the strip is passing through the annealing furnace. However, in such operations, the strip exits the annealing furnace at temperatures approaching 1100.degree. F., which is far above the melting temperature (about 800.degree. F.) of the zinc and its additives. The 1100.degree. F. temperature strip travels in a vertical direction toward the tower roller. As this hot strip wraps around the tower roller at a temperature near or above the melting temperature of the zinc, some of the zinc is deposited on that roller; this oftentimes results in a smearing of the remaining zinc on the face of the strip. This situation has also caused the unacceptable appearance of imperfections on the surface of the strip.
A further problem that has occurred is that as the tower roller becomes smeared with zinc, the tower roller eventually needs to have the zinc removed, such as by grinding, from the roller. Usually the grinding process is performed by an individual who stands next to the roller and uses a hand-held grinder against the surface of the roller. This operation presents significant safety concerns. For example, the individual is exposed to the extreme heat of the traveling strip. Also, because the grinding operation is often performed while the line continues to operate, the roller is rotating at a significant speed, thereby raising the prospect that the individual could be drawn into the roller and injured.
Faced with these problems, the industry has attempted to cool the strip between the annealing furnace and the tower roller by blowing low-pressure, low-velocity air on the strip in large volume and large cross-section from a single air header. By so cooling the strip, the size of the spangle could, in theory, be reduced. Thus, the galvanized steel would be of a better quality for consumer products.
In this prior art design, the large volume, low-pressure, low-velocity air makes contact with the strip in a relatively large area, with the air header being positioned in front and in back of the traveling strip, so that the air stream exiting the header is usually wider than the strip width.
In the cooling approach used by this prior art design, the cooling air first makes contact with the strip as the strip enters the entry end of the cooler. Because the strip at this point is generally about 110.degree. F., the air quickly becomes superheated. The superheated air then forms a boundary layer next to the strip and travels upward with the strip, at the speed of the traveling strip. As will be appreciated, the boundary layer of the superheated air acts as an insulator. Thus, the large volume, low-pressure, low-velocity air being directed toward the strip at points above the entry end is unable to break through the superheated boundary layer air to further cool the strip. Therefore, this method of attempting to cool the strip is inefficient because only the air that first contacts the strip at the entry end is able to perform any significant cooling. Furthermore, this cooling process is energy inefficient because large volumes of air are delivered to the strip that have very little, if any, cooling value past the entry point.
Because the air flow in this prior art approach is directed at the strip at a 90.degree. angle, the air that bounces back from the strip creates a turbulent zone immediately adjacent to the traveling strip. The cooling of the strip is thus further hindered because the edges of the strip naturally cool faster than the center of the strip. This non-uniform cooling of the strip has several disadvantages. First, the edges of the strip may become wavy and the sizes of the spangles could be different across the face of the strip. Second, this cooling process is also inefficient in that the temperature of the strip generally cannot be reduced to a sufficient level where the zinc fully solidifies and "freezes" on the strip prior to wrapping around the tower roller. Consequently, in such operations of the galvanizing lines, the temperature of the strip may be about 700.degree.-800.degree. F., after going through this type of air-cooling apparatus. This high temperature causes the phenomena described earlier, i.e., the molten zinc is deposited on the tower, and sometimes even subsequent rollers, additionally causing a smeared surface on the strip.
With this general discussions of the problems associated with the prior art in mind, it is a nonlimiting object of the present invention is to provide an apparatus for cooling a coated strip of material which does not have the inherent deficiencies of the prior art.
Another nonlimiting object of the present invention is to provide a cooling apparatus which reduces the temperature of the strip from approximately 1100.degree. F. to about 300.degree. F. prior to contacting deflector rollers.
Yet a further nonlimiting object of the present invention is to provide a cooling apparatus which harvests the heat energy from the superheated air deflecting away from the strip to generate steam.
These nonlimiting described objects and the other objects and advantages of the present invention will become apparent to those skilled in the art with reference to the foregoing, the attached drawings, and the description of the invention which hereinafter follows.